Another strong clue supporting Marcionite priority: the "disciples of John" are really Christians

[schillingklaus]

A John just invented as a tool for making Jews believe in Jesus as fulfiller of Scripture cannot be deemed independent.

[Giuseppe]

John enters in the Christian Gospel tradition only the first time when the expression "disciples of John" appears. But a such expression is polemical.

Evidence that similar expressions "disciples of X, of Y, of Z" are polemical, or born in a polemical context, is 1 Corinthians 1:12:

Now I mean this, that each one of you is saying, “I am with Paul,” or “I am with Apollos,” or “I am with Cephas,” or “I am with Christ

Just as one can't deny that Cephas or Paul or Apollos were Christians, so one can't deny that John was a Christian.